The invention relates to a diecasting method for producing cast pieces which are low in gas, pores and oxides, particularly cast pieces of 1-3 kg shot weight of metals or their alloys, such as aluminum and aluminum alloys or the like, by means of a preferably horizontal cold chamber diecasting machine of known design, with the metal being transported by means of a vacuum from the melting or holding device (holding furnace) into the fill chamber through a suction pipe, and the diecasting mold also being maintained under vacuum.
The diecasting process for aluminum is a very economical method of producing parts, even parts having complicated shapes, in but a few process steps.
However, cast pieces produced in conventional diecasting processes have structural regions which, as a result of the process, are loosened, porous and full of impurities, resulting in a lack of strength and bubble formation during heat treatments. The utilization of the characteristics attainable in aluminum cast materials by way of required heat treatment methods, such as, for example solution heat treatment, is not possible because of these phenomena.
To realize an improvement in quality, the so-called pore-free process has been developed (German Pat. No. 1,558,261). Here, a displacement gas, preferably oxygen, is introduced into the fill chamber and into the mold cavity and thus the air is displaced. Thereafter, the liquid aluminum is filled into the fill chamber and the turbulent interaction of the liquid aluminum with the oxygen produces a reaction to form aluminum oxide particles which are then present in the cast piece as dispersed solid particles. Although the cast pieces are heat treatable and of good quality, the process has the drawback that only lubricants which are free of mineral oil can be used since otherwise there would exist the danger of explosion during the filling phase. This fact produces difficulties in the distribution of the lubricant since inorganic solid lubricants are used with preference. The process steps of rinsing and applying the lubricant requires much time in the casting cycle so that the production output in this process is not very high. Moreover, cast pieces produced according to this process are distinguished by a high oxide content. With insufficient mold filling rates, there may occur fluctuations in the oxide distribution which adversely influence the quality of the cast pieces.
In order to realize increased yield of cast pieces, diecasting processes have been developed in which the metal melt is pulled into the fill chamber through a suction tube by means of a vacuum (DE-OS No. 1,458,151). In this case, a sufficient vacuum must exist during the mold filling phase and during the dosaging phase so that the hydrogen content of the melt and the air present in the mold cavity and in the fill chamber as well as the casting gases developed during contact with the liquid aluminum can be attracted. This process is directed toward reducing the oxide content in the cast pieces.
A further prior art process (Periodical Giesserei 64 (1977), No. 9, pages 236 et seq.) operates with a very short vacuum period of about 1.5 seconds. Although it is possible in this case to extract, for example, the air and the first casting gases, the period of dwell of the vacuum is not sufficient to perform sufficient degasification during the casting process. The result is that during the mold filling phase a considerable amount of gas and impurities are still enclosed in the cast pieces. As a consequence, the cast pieces produced according to this method cannot be heat treated at high temperatures since bubble formation will then be noted.